In general, one or more of the magnetic disks used in a hard disk drive (HDD) is provided with a plurality of data tracks and a plurality of servo tracks. Each servo track includes a plurality of servo sectors having address data. Moreover, each data track includes a plurality of data sectors containing user data. Data tracks are recorded between servo sectors separated in the circumferential direction.
The HDD is provided with an oscillating actuator, and a head slider is supported on this actuator. The HDD reads out the address data of the servo sector using the head slider, and controls the actuator with this address data. As such the HDD is able to move the head slider to the proper radial position, and then position the head there. Once positioned on the target data track, the head slider reads out data or writes in data to the target data sector within this track.
In the data readout process, signals read out from the magnetic disk by the head slider are subject to the designated signal processing such as waveform shaping or decryption by the signal processing circuit, and transferred to the host. Data transferred from the host is written into the magnetic disk by the head slider after receiving the designated processing by the signal processing circuit in the same way.
As described above, control of the positioning of the head slider is carried out using servo data on the magnetic disk. The servo sector includes the cylinder ID, sector number, burst pattern and the like. The cylinder ID indicates the track address while the sector number indicates the sector address within the track. The burst pattern has data on the position of the magnetic head relative to the track. The servo track is comprised of a plurality of servo sectors separated in the circumferential direction, and sectors which span all the servo tracks have their positions aligned in the circumferential direction.
Servo data are written into the magnetic disk in the factory before the HDD is shipped as a product. At present the process of writing in the servo tracks occupies a significant part of the manufacturing cost of the HDD. Self-servo writing (SSW) uses the mechanical mechanism of the HDD itself as a mechanism for servo writing, controlling the spindle motor and the voice coil motor within the HDD from an external circuit using the external circuit to write in the servo patterns. This enables the servo track writer (STW) to be eliminated, contributing to a reduction in HDD manufacturing costs.
SSW uses the fact that the positions of the read element and write element in the head element unit differ in the radial direction. This separation is known as the read/write offset. Thus the self-propagation of servo sectors into which new tracks are written presumes that the precision of the servo sectors read out in the radial direction will be carried on unchanged in the new newly written servo sectors. However, due to various factors that cause errors to occur, this precision deteriorates in the process of propagation. Deterioration in the precision of positioning causes discrepancies in the proper position for a servo sector, and this is carried on in subsequent propagations.
Moreover, this process also depends on the characteristics of the servo loop that enables the head element unit to execute track following. Deterioration in precision is a complex mechanism involving a number of factors. In this way track shape errors accumulate in regions with frequencies where the gain of the closed loop transfer characteristics exceeds 1 due to repeated propagation and writing operations during SSW.
With conventional techniques, the operation control program has to measure and model the closed loop characteristics for the servo system in advance. However, the actual transmission characteristics for the magnetic disk differ slightly due to a variety of causes. As a result, there can be a discrepancy in characteristics between the servo system model and the actual servo system. In particular, it is very difficult to accurately acquire transfer characteristics for low-frequency regions in the vicinity of the first-order component of magnetic disk rotation. As a result, SSW cannot apply an appropriate correction to the servo system, and track shape errors can grow with repeated propagation and writing.